But there are other agencies at work as transformers on and within the earth’s crust. There are in most rocks a series of divisional planes, which may be either vertical or inclined, and to which the name of joints has been given. These may arise from various causes. Both in sedimentary and igneous rocks they are in part due to contraction during consolidation—in the former when they lose their contained water, in the latter when they solidify from a molten condition. Joints may also be called into being by the effects of internal pressures and movements within the earth’s crust, such structures having been experimentally reproduced by Daubrée in materials under stress by torsion and by simple pressure. The granite of Aswan displays such jointing to a marked degree, giving rise to remarkable hills composed of huge boulders of granite piled on one another. V.—CHEMICAL TRANSFORMATION OF ROCKS. Besides the mechanical effects of river, rain, and wind, other changes whose wide-reaching significance cannot be over-estimated, are taking place on and below the earth’s surface. Chemical action is slowly at work producing effects of the first importance to man. Rain-water has the power of absorbing important quantities of carbonic acid gas and oxygen from the atmosphere. On the average, rain-water contains 1·77 per cent by volume of dissolved carbonic acid gas, and 33·76 per cent of dissolved oxygen. In passing through the soil, rain-water also absorbs the organic acids formed by the decomposition of plant remains. These dissolved gases and organic acids render rain an active chemical agent in the alteration of rocks, its effects being conveniently classified under the headings: (1) Oxidation; (2) Solution; (3) Formation of Carbonates; and (4) Hydration. (1) Oxidation results in the formation of thin crusts on the surface of rocks, the compounds of manganese and iron so frequently present in them being also rusted or hydrated by the action of the rain-water. Nothing is more striking than the presence of the dark films on the desert limestones in regions which are liable to a certain amount of rainfall, and nothing more convincing as to their origin than their absence in those portions of the south-western desert of Egypt where rain is of great rarity. Near the Nile, the Red Sea and the Mediterranean, dew may take the part of rain in action, and in a sense the results of its activity may appear more intense, as rain is liable to wash away the products of its own handiwork.